2 cups port wine or 1 cup grape juice and 1 cup reduced-sodium beef broth
2 tablespoons balsamic vinegar
1 teaspoon butter
1/4 teaspoon salt
1/8 teaspoon pepper
Combine the rosemary, garlic, salt and pepper; rub evenly over tenderloin. Cover and refrigerate for 2 hours. Place meat on a rack in shallow roasting pan. Bake, uncovered, at 400° for 50-70 minutes or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Let stand for 10 minutes before slicing.
In a saucepan, bring wine or grape juice and broth to a boil; cook until reduced to 3/4 cup. Add vinegar; cook for 3-4 minutes or until reduced to a sauce consistency. Stir in butter, salt and pepper. Serve with tenderloin.